**4. Conclusions**

262 Biogas

The untreated "raw" dairy wastewater with low value of pH (4.27) was completely nonactive in hydrogen generation by microbiological method. However, we assumed that the same wastewater under controlled pH can generate hydrogen similarly as a sterilized one. Therefore, in order to achieve similar conditions like in bioreactor operating under controlled pH we performed our batch tests in small photoreactors (capacity of 60 ml with working capacity of 30 ml) correcting pH with 0.5M solution of NaOH every 12 h. Medium containing non-sterilized dairy wastewater with concentration of 40 v/v % was inoculated with bacteria at two different concentrations: 0.086g dry wt/l (10 vol.%) or 0.36 g dry wt/l (30 vol.%). Data presented in table 5 indicate that stabilization of the system at pH close to 7 allows for hydrogen generation even from the untreated dairy wastewater. Application of inoculums with concentration at the 0.36 g dry wt/l level generates 3.6 l H2/l. The four-fold dilution of microorganisms reduces the volume of hydrogen to 2.6 l H2/l. Although the starting time was relatively long (about 20 h) savings which could arise from the application of untreated waste can be significant. Performing the same experiment with brewery waste II ( concentration 40 v/v % ) the yield of the generated hydrogen has not been improved. In this case the value of pH rapidly grew to 7.5- 7.9 in the first two days. However , it can not be excluded that in the system with controlled pH this yield could be much higher. Preliminary experiments performed under such

**3.5 The influence of pH correction on hydrogen production** 

conditions confirm this assumption.

**3.6 Kinetic of hydrogen generation** 

reaction rate and prolongation of the lag phase.

Rmax,H2 (l/l/h)

0.038±0.005 0.056±0.009 λH2 (h)

20.7±3.6 17.0±4.5 Y (l H2/l waste)

> 6.0 8.6

Table 5. Kinetic parameters of cumulative hydrogen production for non-treated 40 % dairy wastewater, with correction of pH for different concentration of inoculums (Seifert, 2010).

The results presented in this section suggest that hydrogen generation can be effectively performed under solar radiation in photobioreactor operating under continuous

The results of kinetic considerations based on modified Gompertz equation (Eq. 4) are shown in table 6. Independently from the kind of food waste (in the active of concentration) it was observed that the increase of the volume of generated hydrogen, small drops in

These results showed that higher substrate yield increases the reaction rate. Moreover, these values are well correlated with the lag phase in systems with higher concentration of wastes

are caused probably by longer adaptation of microorganisms to the bed.

pH final

> 6.8 6.7

COD loss (g O2/l)

> 3.8 4.6

COD loss (%)

> 20 23

Biomass (g/l)

> 2.2 2.8

Hmax (l/l)

2.58±0.16 3.62±0.24

\* expressed in g dry wt/l \*\* biomass increase

Inoculum conc.

> 0.086 0.36

conditions.

The presented results shows that the waste studied in this paper represent a vary good substrate in photophermentation by *Rhodobacter sphaeroides.* Light intensity of 9 klx and inoculum concentration of 0.36 g dry wt/l (30% v/v) were used as the most effective (high light conversion efficiency and short duration of the process). The studied wastes has to be treated with high temperature (20 min in 120oC). This pretreatment significantly increases H2 production. The optimum concentrations of wastes were estimated: 40% v/v for dairy waste and 10% v/v for brewery waste with high COD. These wastes represent the effective (comparable with L-malic acid) nutrient for hydrogen production. Higher wastes concentrations inhibit the process as it initiate fermentation which starts to compete with hydrogen production and additionally increases NH4 + concentration, which also negatively affect the process. Brewery waste with low COD shows low efficiencies and needs to be concentrated to supply sufficient concentration of organic compounds. An application of untreated dairy wastewater containing suspensions in efficient hydrogen generation process can be performed only at controlled acidity (pH = 7.0). Kinetic measurements proved that the rate of hydrogen generation drops with concentration of the waste and prolongs the lag phase.
